feat: adds a constraint counter api

[gbotrel]

Usage, inside a circuit:

	start := api.Tag("start_mulfp2")

	l1 := api.Add(e1.A0, e1.A1)
	l2 := api.Add(e2.A0, e2.A1)
	// ... etc

	end := api.Tag("end_mulfp2")
	api.AddCounter(start, end)

Then access through CompiledConstraintSystem with

ccs, _ := frontend.Compile(...)
counters := ccs.GetCounters()

// in a test file
assert.SolvingSucceeded(&circuit, &witness, test.WithCurves(ecc.BW6_761))
counters := assert.GetCounters(&circuit)

for _, c := range counters {
t.Log(c)
}

Counter struct:

// Counter contains measurements of useful statistics between two Tag
type Counter struct {
	From, To      string
	NbVariables   int
	NbConstraints int
	CurveID       ecc.ID
	BackendID     backend.ID
}

displays as:

groth16[bn254] start[g1.go:210] - end[g1.go:220]: 2277 variables, 2508 constraints

Notes:

• measurement for PlonK is contextual, since linear expression reduction may use previously reduced expression (ie measuring two repeating piece of circuit may yield a smaller number of constraint the second time)
• api naming is not definitive

Also, first steps for #174

[gbotrel]

• need a better way to identify Counter objects. From / To will (currently) include line number, bad idea if we want to compare counters across branches for non-regression for example.
• figure out feature-wise, if maybe we want to have a default counter (englobing the whole circuit). What other stats we want to put in there. If we want to make that available only with a build tag.